Plasmin cleaves the heavy (H) chain of some IgG molecules in the hinge region on the N-terminal side of the inter-H chain disulfide bridges. Cleavage of one H chain produces a Fab fragment and an Fab/Fc fragment; cleavage of the remaining H chain in the Fab/Fc fragment can then produce another Fab fragment and an Fc fragment. We examined the kinetics of fragmentation by HPEC analysis, and by SDS PAGE, of the products formed upon incubating IgG (100 mg/ml) with plasmin (1 mg/ml) at 25 degrees C, pH 7.4, for various periods of time. Because the IgG consists of 4 different subclasses, having different susceptibilities to cleavage by plasmin, analysis of the kinetics is complex. The analysis suggests at least three different populations of molecules: one (IgG2) is totally resistant to cleavage, one (IgG4) is fragmented at a very low rate, and one (IgG1 and perhaps IgG3) is fragmented rapidly. Even at very high levels of enzyme and long incubation times, a small amount (ca. 5%) of the intermediate "Fab/Fc" fragment persists. It is possible that some of this material may in fact be (Fab')2 fragments, derived by cleavage of both H chains at a point between-two inter-H chain disulfides so that the two Fab arms remain covalently joined by one or more disulfide bridges. From the primary structures of the hinge regions of IgG subclasses, it would appear that IgG3 might undergo such cleavage leading to (Fab')2 and Fc fragments. This possibility will be explored by examining the cleavage products of purified IgG3 with plasmin.